Components of conventional walk behind engine-driven lawn mowers include engine, rotary blade, wheels and handle. For security and grass collection reasons, the cutter, i.e. rotary blade, is arranged in a housing which is of essentially cylindrical shape.
The various components are mounted on a deck, i.e. the lawn mower chassis. On some lawn mower types, the cutter housing form part of an exposed or hidden deck, onto which external components, such as handle and wheels, are fastened.
A completed deck is usually provided with outer fastening surfaces that define suitable securing points for external components, and an inner surface that defines the cutting blade housing. The fastening surfaces for external components such as wheels and handle are often vertical for this purpose.
A common manufacturing method for lawn mower decks is to mould a sheet material into a suitable shape, by for example deep drawing.
According to a known manufacturing method, an external surface comprising the fastening surfaces is first formed from the sheet, and the cylinder shaped cutting blade housing is subsequently arranged inside by welding.
According to another known manufacturing method, the inner surface defining the housing is formed first, by form pressing a sheet blank, and the necessary outer fastening surfaces are provided subsequently. A known method for providing the fastening surfaces is to contour cut the sheet blank into a shape that includes both the form pressed housing, and flat surfaces which can be perpendicularly angled so as to form vertical fastening surfaces suitable for securing wheels and/or handle to the deck.
A lawn mower deck is subject to stress during use, especially at the securing points between the deck and wheels and/or handle. This puts the deck at risk for deformation and thus for malfunction.
To reduce this risk, it is known to reinforce a deck by arranging additional strengthening bars between the fastening surfaces and the cutter housing. The strengthening bars are usually welded to the deck, since this provides a more stable construction than does for example screwing.
The methods described above, however, suffer from several disadvantages.
Changing the shape of the sheet material, by for example form pressing, results in tensions in the material. Welding is associated with heating the material, which can release the tensions and cause deformation of the deck. Since, to make the lawn mower rest on all wheels, levelling of the wheels is crucial for the construction, even a slight change in the geometry of the deck might cause the wheel positions to be altered so that the deck must be rejected.
The need for welding extra components onto the sheet material also puts limitations as to what sheet material can be used. Galvanized sheet, for example, complicates welding, but would in many other aspects be well suited for the purpose of making a lawn mower deck. Moreover, galvanized sheet needs to be galvanized again after additional components have been fastened thereto by welding.
The need for fastening additional components also increases production costs, measured both in money, time and manufacturing complexity.
The additional components further result in problems with tolerances, because of long tolerance chains between different areas of the deck structure.